Fast Blow Circuit Breaker
Fast Blow Circuit Breaker
Puzzled by some odd behaviour in a circuit breaker, and hoping the forum can help shed some light.
We have a troubleshooting test box that relies upon fast-blow fuses to give an indication that a circuit functions correctly.
The cables from the test box are connected in series with a cable harness on the aircraft to "see" if an electric pulse is delivered.
The user knows that the circuit works, from power supply to ground, when the test-box fuse blows.
The fuses in the test box take the place of squib cartridges which activate the fire extinguishing bottles (engine compartment/cargo compartment/etc.)
Obviously, you don't want to discharge the fire extinguishers in the hangar.
To do the test, we disconnect the cables from the squibs, cover the squibs, and connect the leads to the test box.
When the technician pushes the button in the cockpit, the fuse in the test box should blow, simulating a firing of the extinguisher cartridges.
Works well enough, but you have to remember to bring fuses when you go to the aircraft, the aircraft may not be parked in a nice cozy hangar, it might be nighttime, -30 degrees, etc... you get the idea.
I came up with a plan to improve the test box by replacing the fuses with fast-blow circuit breakers.
I had encountered TE (Potter-Brumfeld) fast circuit breakers on a previous project, and I guess I was hoping to make myself a hero on this one!
A circuit breaker would be easier to see trip, and less hassle to reset it. Fine in theory, but we never could get one to trip.
Firing circuit: Supply voltage 28V DC, 10A pulse for 50 millisecond.
Total circuit resistance (incl very long wires) 1.8 Ohm.
Squib cartridge load resistance 1.0 ohm.
Fuse p/n 22500002-HXP (Littelfuse)
Fuse rating 2 Amp
Fuse resistance = 0.05 Ohm
Dummy load (in series with fuse) 0.1 Ohm
Current pulse with fuse = 28V / (1.7+0.1+0.05) = 15 Amp
Fuse trip time 30 millisecond max @ 10A - which is why it works to detect the 50 ms pulse.
Circuit breaker Potter+Brumfeld (TE) W67-X2Q50-2
CB rating 2 Ampere
CB Resistance = 0.28 Ohm
Current pulse with CB = 28V / (1.7+0.1+0.28) = 13 Amp
CB Trip time 12 millisecond max @ 13 A - which should work well, but it didn't.
Replaced the first CB we tried, then the next, checked DC polarity... nothing could make it trip.
Eventually we got frustrated enough that we took the CB out, put it on a regulated 28V power supply, cranked it to 10 Amps, and still nothing tripped, even leaving it on for several minutes.
I think we returned 8 CB's back to the supplier, in the end. All apparently defective.
By the way, nothing's wrong with the aircraft. An all-up test of the bottles was conducted successfully last month.
The purpose of the test box is to allow the customer to test the system in the future, without firing the extinguishers.
The customer will be satisfied with using fuses, they don't know what they're missing.
Any ideas what went wrong?
I have a far-out theory, but I'd like to hear some other perspectives before biasing anyone's opinion.